The New England Primate Center maintains a colony of approximately 180 common marmosets, Callithrix jacchus. Seventeen animals from this colony exhibit clinical signs compatible with a disease of abnormal hemostasis. Clinical histories in these animals include excessive bleeding following trauma, marked hematoma q aneurism formation and bruising following venipuncture, excessive postpartum hemorrhage, gastrointestinal bleeding, and epistaxis. Severity of hemorrhagic episodes in some instances necessitates whole blood transfusions. Examination of the genealogies of affected animals revealed a familial bleeding tendency, and all affected animals were found to be descendants of animals introduced into this colony from one other breeding colony. These findings suggested the presence of a heritable coagulopathy in the common marmoset. Work is being conducted to identify the etiology of this apparent coagulopathy in the hopes that a breeding colony of affected animals can be established to serve as an animal model. Complete blood counts and prothrombin times were normal in all animals. Ten animals had elevated partial thromboplastin times (r40 sec.) which suggested a possible clotting factor deficiency in the intrinsic pathway. Coagulation factor activities to include Factors VIII, IX and XI, and Von Willebrand factor antigen and functional Von Willebrand factor (ristocetin cofactor activity assay) were measured. No significant differences were identified between affected and control animals. Current work is directed toward evaluation of platelet function. Affected and control animals will be screened using an in vivo bleeding time test and the dilute whole blood clot retraction and lysis test. As bleeding times have never been reported in Callithrix jacchus, methods are being developed to measure bleeding times in this species. After the technique is validated, bleeding time measurements in normal control marmosets will be compared to affected animals. Dependent on results of screening tests for platelet dysfunction, specific platelet function studies (platelet aggregometry) will be performed.